Move contributing guidelines and fix build instructions (#620)

* Move contributing guidelines to CONTRIBUTING.md

* Update build instructions

* Update PR template for new contributing guidelines

* Fix contributing guidelines reference in README

.github/CONTRIBUTING.md

@@ -1 +1,28 @@
-Please refer to the **Contributing** and **Contributing Guidelines** sections of our [readme](https://github.com/privacytoolsIO/privacytools.io/blob/master/README.md) for the most up-to-date guidelines. Thank you.
+# Contributing Guidelines
+
+Please read this before contributing.
+
+## Rules
+
+- Be nice and respectful.
+- English only.
+- Be constructive.
+
+## Quality over Quantity
+
+We're trying to keep it simple and promote the best tools, not all of them. Maximum are three software choices, but it's possible to add more under "Worth Mentioning" headlines. ([example](https://privacytoolsio.github.io/privacytools.io/#im))
+
+## Software Criteria
+
+- Open Source
+- Cross-platform
+- Easy to use
+- Privacy respecting
+
+There can be exceptions if no software is available that meet the criteria.
+
+## Images
+
+- Provider logos should be exactly 200 x 70 ([example](https://www.privacytools.io/assets/img/provider/AirVPN.gif))
+- Tool logos should be exactly 120 x 120 ([example](https://www.privacytools.io/assets/img/tools/ChatSecure.png))
+

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,4 @@
-<!-- PLEASE READ OUR [CONTRIBUTING GUIDELINES](https://github.com/privacytoolsIO/privacytools.io#contributing-guidelines) BEFORE SUBMITTING -->
+<!-- PLEASE READ OUR [CONTRIBUTING GUIDELINES](https://github.com/privacytoolsIO/privacytools.io/blob/master/.github/CONTRIBUTING.md) BEFORE SUBMITTING -->
 
 ## Description